The present invention relates to a glaze for coating ceramic superplastic forming (SPF) dies to provide a hard forming surface to increase life.
Plasma spraying is becoming more widely accepted for industrial use. The plasma spraying process consists of introducing a powder carried by a gas into a high temperature flow of ionized gas. The powder particles are subsequently melted or partially melted and propelled towards a substrate by the carrier gas. Upon impact with the target surface, the molten particles form lamellae or splats that adhere to the substrate by mechanical bonding to surface imperfections. Many metal, ceramic, and polymer powders are commercially available for plasma spraying and other thermal spray processes. Commercial powders are often spherical or semi-spherical in shape and within a specific size range, primarily to allow for good flowability or feeding characteristics. Plasma spraying can deposit a wide range of materials onto a wide range of substrates. Nearly any material that can be produced in powder form can be deposited by plasma spraying.
Plasma spray coatings made from zircon (ZrSiO4) produced either tetragonal or monoclinic zirconia (ZrO2) or amorphous silica (SiO2). The tetragonal coating particles must have been heated above about 1200xc2x0 C. while the monoclinic particles never reached 1200xc2x0 C.
Water-stabilized plasma (WSP) spraying of angular garnet (crystalline) and basalt (amorphous) powders in the range of 56 to 200 xcexcm found that the sprayed materials were well spherodized, lower in silicon and alkali content, and in the amorphous state. The extraordinarily high temperatures ( greater than 10,000xc2x0 K) reached in such a plasma probably allowed for melting and spheroidization of the powders while volatilizing some constituents. The amorphicity of the coatings was likely influenced by the rapid solidification of the splats, estimated to be on the order of 10 xcexcs. Most coatings were found to have extremely low spherical or pseudospherical porosity, which ranged from 4.6% to less than 2%, which indicated that the glassy state of the impacting particles allowed for high flowability. The residence time of the powders in the plasma was the dominant factor with respect to coating quality while the spraying distance was determined to be of much less consequence.
Another study focused on the adhesion of plasma sprayed borosilicate glass as a function of the substrate (steel) preheat temperature. Coating thicknesses of 200 to 800 xcexcm with approximately 5% porosity were obtained by feeding extremely angular powder. Substrate preheat temperatures ranged from 50xc2x0 C. to 700xc2x0 C. The plasma plume was scanned across the substrate surface prior to introducing powder into the system. The coating adhesive strength increased with substrate preheating up to Tg of the glass.
The substrate temperature greatly influenced the adhesion of such coatings by allowing the molten glass particles to remain less viscous for a longer time. From analysis of coating morphologies, the lamellae were better able to completely fill surface anomalies and to bond better mechanically to the substrate. Assuming that the lamellae have a much greater diameter than thickness, a one-dimensional heat conduction model for the coating is described by:
k/(xcfx81xc2x7Cp)xc2x7∂2T/∂x2=∂T/∂txe2x80x83xe2x80x83(1) 
where T is the coating temperature at a distance x from the substrate at a time t and k, p, and Cp are the thermal conductivity, density, and specific heat of the droplet. The presumption that preheating the substrate yields increased adhesion is strengthened by finding that greater deposition efficiency, or splat retention, was also obtained. Not only does substrate preheating allow for better glass flow over the substrate surface, but lower thermal gradients and tensile stresses arise across the thickness of the cooling splats.
A simplistic account of the maximum stress, "sgr"c, that arises in such a coating is depicted by the relationship:
"sgr"c)=xcex1cxc2x7(Tgxe2x88x92Tb)xc2x7Ecxe2x80x83xe2x80x83(2) 
where xcex1c, Tg, and Ec are the coefficient of expansion, glass transformation temperature, and elastic modules of the glass, respectively, and Tb is the substrate preheat temperature.
Rare-earth alumino-silicates, yttria-alumino-silicates, and the addition of lanthanides to more common glass systems have also been studied. Yttrium is not a rare-earth, but closely emulates true rare-earths when included in alumino-silicates. Rare-earth alumino-silicate glasses are of both scientific and industrial interest due to their relatively high Tg, high refractive indices (nd), high hardness, high elastic moduli, chemical durability, and moderate thermal expansion.
Rare-earth ions do not play a primary role in glass formation, but significantly modify the properties of traditional glasses. Lanthanum additions (as the oxide) to sodium silicate glasses account for increased Tg, nd, and density (xcfx81) while lowering the thermal expansion and electrical conductivity. While lanthanum additions increase Tg regardless of whether soda or silica is replaced, the coefficient of thermal expansion is more markedly reduced by replacement of soda. The mechanism may involve each trivalent lanthanum ion acting as a modifier by creating three nonbridging oxygens, thus explaining the increase in Tg. Another option assumes that lanthanum enters a xe2x80x98networkxe2x80x99 site, likely octahedral rather than tetrahedral, because of its large ionic radius, and simultaneously increases the connectivity of the structure while decreasing the concentration of nonbridging oxygens.
The maximum solubility limit of rare-earth ions that can be incorporated into most rare-earth alumino-silicate glass structures increases with decreasing atomic radii and decreasing atomic number from La to Yb. This trend is a function of lanthanide contraction. In the system xLa2O3-25Al2O3-(75-x)SiO2, devitrification begins between 25 and 30 mole % La2O3. All glass compositions are given in mole ratio form unless otherwise noted. Glass transformation temperature, thermal expansion, and nd have all been found to be dependent upon ionic radius (in the system 20La2O3-20Al2O3-60SiO2). The transformation temperature increased with decreasing ionic radius while both thermal expansion and nd decreased. These property variations apparently are at least partially the result of rare-earth ion field strength rather than ion size alone.
Although we evaluated only one glass composition (8.25La2O3-19.25Al2O3-72.5SiO2), Beinarovich et al. discovered that glass formed at approximately 1290xc2x0 C. in the synthetic batch, about 150xc2x0 C. lower than in the traditional oxide batch. xe2x80x9cTraditional oxide batchxe2x80x9d refers to glasses melted from fine raw material oxides while the xe2x80x9csynthetic batchxe2x80x9d refers to ions suspended in solution. The synthetic batch has higher constituent dispersion and begins with amorphous components.
Karlsson conducted a three-part study on the crystallization behavior of various La2O3xe2x80x94Al2O3xe2x80x94SiO2 glasses and measured some physical properties of the glassy state. The glasses of the first study melted at 1500xc2x0 C. and were cooled slowly to room temperature, after which large, white crystals were observed within the glass. X-ray analysis showed these crystals to be orthorhombic La2O3-2SiO2 grown along the a-axis. From the X-ray patterns the lattice parameters of the devitrified phase were determined as a=13.15 xc3x85, b=10.15 xc3x85, and c=8.64 xc3x85. A density of 2.58 g/cm3 was calculated for this phase assuming four molecules per unit cell. Its melting temperature was observed to be 1420xc2x110xc2x0 C.
The second Karlsson study involved devitrification products of glasses slightly higher in alumina content. These glasses were heat treated at 950xc2x0 C. and 1200xc2x0 C. after which small, white, crystalline whiskers were observed in the glass. X-ray patterns of these crystals showed the compound to be orthorhombic 2La2O3-2Al2O3-5SiO2 with lattice parameters of a=15.02 xc3x85, b=12.97 xc3x85, and c=xc3x85. This phase could be hexagonal-like since a/b only deviates by 3.5% from 23/3. A melting point of 1280xc2x110xc2x0 C. was determined by high-temperature microscopy. Again assuming four molecules per unit cell, a density of 4.46 g/cm3 was calculated. The 2La2O3-2Al2O3-5SiO2 phase devitrified from some glasses and melted incongruently. Some glasses produced La2O3-2SiO2 while others yielded xcex2-alumina and La2O3-7SiO2. Karlsson did not find La2O3-11SiO2, but its reported formation conforms well to the devitrification scheme. Glasses along the peritectic line to the left and below the peritectic point were found to produce the 2La2O3-2Al2O3-5SiO2, and La2O3-2SiO2 phases along with minute hexagonal crystals with a +7.76 xc3x85 and c=11.28 xc3x85. A composition of La2O3-3SiO2 was suggested for these crystals which melted at 1400xc2x110xc2x0 C.
The third Karlsson study involved physical property determinations of the glasses listed in Table 1. These glasses were melted at 1550xc2x0 C. and annealed at 800xc2x0 C. Additions of La2O3 to all of the glasses slightly increased their CTE with the CTE doubling at the 2La2O3-2Al2O3-5SiO2 composition.
Table 2 provides some physical properties for these glasses.
Chang et al. studied effects of numerous nucleating agents on devitrification of 18La2O3-24Al2O3-58SiO2 glass. Various oxide and fluoride additions were made to this glass to instigate crystallization after heat treatment between Tg and the onset of crystallization Td. This glass was extremely difficult to crystallize. Devitrified phases were binary or ternary phases containing the nucleant additions. Upon devitrification the coefficients of thermal expansion decreased, often by more than half, and hardness increased by approximately 10%.
Replacement of SiO2 by additions of SnO2 in excess of 4 mole % and P2O5 in excess of 2 mole % caused devitrification in the host glass. Yttria additions stabilized the vitreous state regardless of the presence of other nucleant additions. When 4 mole % CaO displaced SiO2 in conjunction with 12 mole % TiO2, ZrO2, or P2O5 displacement of La2O3, devitrification occurred with the resultant crystalline phasing being anorthite (CaAl2Si2O8). Displacement of La2O3 by 6 mole % each of CaO and CaF2 or 6 mole % each of BaO and BaF2 yielded crystalline phase of anorthite and gehlenite in the case of calcium and celcian in the case of barium. X-ray analysis showed the fluoride additions to be more effective at inducing devitrification. Also of 4 wt % As2O3 resulted in less fragile glasses. In the case of 9 mole % each of MgO and MgF2 displacement of La2O3 coupled with the addition of 1 wt % As2O3, cordierite crystallized upon heat treatment. Overall, when contrasted to those of the amorphous, the physical properties of the corresponding glass-ceramic compared as follows: density remained relatively constant, nd decreased slightly, hardness increased significantly, and thermal expansion decreased by approximately half.
Tables 3 and 4 show properties of lanthanum-alumino-silicate glasses.
Watanabe and Giess studied the devitrification behavior of stoichiometric 2MgO-2Al2O3-5SiO2 (2:2:5). They determined Tg, Td, and Tc of this composition to be approximately 765xc2x0 C., 945xc2x0 C., and 985xc2x0 C., respectively. The CTE of this 2:2:5 glass composition was 20xc3x9710xe2x88x927/xc2x0 C.
Herman et al. sprayed fused, cast, and comminuted 2:2:5 cordierite powder onto a steel substrate. The subsequent coating was found to be amorphous by X-ray diffraction, likely because of high cooling rates (xcx9c106xc2x0 C./s) and the presence of silica. DTA analysis of the crushed coating revealed a transition to metastable xcexc-cordierite, or quartz solid solution, at approximately 840xc2x0 C. and a further transition to high-cordierite at approximately 1060xc2x0 C.
A second Herman study involved thermally-induced spalling of plasma sprayed 2:2:5 cordierite, spinel (MgOxe2x80x94Al2O3) and forsterite (2MgOxe2x80x94SiO2) onto an 80Ni-20Cr bond coat on a steel substrate. Coefficients of thermal expansion for five materials were determined to be 16.7xc3x9710xe2x88x927/xc2x0 C., 76.8xc3x9710xe2x88x927/xc2x0 C., 110xc3x9710xe2x88x927/xc2x0 C., 147xc3x9710xe2x88x927/xc2x0 C., and 135xc3x9710xe2x88x927/xc2x0 C., respectively. The three-layerd specimens, including the substrate, were then heated by a flame impinging on the surface of the top coating followed by compressed air cooling, resulting in an average rate of temperature change equal to 4.3xc2x0 C./min. The maximum coating temperature reached during the heating stage was approximately 1025xc2x145xc2x0 C. Thermal stress within the coatings was estimated by the relationship:
"sgr"1=Exc2x7xcex94xcex1xc2x7xcex94Txc2x7(V2+V3)/(1xe2x88x92xcexc)xe2x80x83xe2x80x83(3) 
where E is the elastic modululs of the system, xcex94xcex1 is the thermal expansion mismatch, xcex94T is the rate of change in temperature, V2 and V3 are the volume fractions of the bond coat and substrate, and xcexc is Poisson""s ratio. If strain is uniform throughout the coating-bond coat-substrate system, then E can be calculated by:
E=E1xc2x7V1+E2xc2x7V2+E3xc2x7V3xe2x80x83xe2x80x83(4) 
where subscripts 1, 2, and 3 represent the top coat, bond coat, and substrate, respectively. Because of increased thermal stresses with greater CTE mismatch, the cordierite was first to spall followed by spinel, then forsterite.
Tables 5, 6a, and 6b show properties of 21 different lanthanum-alumino-silicate glass-ceramics.
Two systems of primary interest for hardening the surface of ceramic SPF dies according to the present invention are the cordierite and lanthanum-alumino-silicate glasses and their related glass-ceramics. Both systems have moderate to low coefficients of thermal expansion (CTE) in both their amorphous and devitrified states, high chemical suresistance, good strength, and high glass transformation temperatures (Tg). These attributes make the two systems attractive as coatings for high temperature applications involving thermal cycling if paired with substrate materials of similar expansion characteristics.
Coating materials based upon cordierite (2MgO-2Al2O3-5SiO2 or 2xe2x80xa22xe2x80xa25 MAS) with titania additions have a coefficient of thermal expansion that can be tailored by composition or thermal treatment to match each of the refractory concretes commonly used to make ceramic SPF dies. A lanthanum-alumino-silicate (LAS) system may be an alternative to the preferred 2xe2x80xa22xe2x80xa25 MAS system, but at a higher cost.
Cordierite or magnesia-alumino-silicate glass is attractive as a thermal shock resistant coating material for ceramic SPF dies because of its low thermal expansion. Hummel and Reid showed that higher alumina contents rather than magnesia were more effective in lowering thermal expansion with constant silica. Of the five compositions they studied (1360xc2x0 C. eutectic, 1345xc2x0 C., 1:1:4, 1:1:6, and 1:1:10), the MgOxe2x80x94Al2O3-10SiO2 (1:1:10) glass had the lowest thermal expansion, 20.2xc3x9710xe2x88x927/xc2x0 C. The thermal expansion of solid solution cordierite of the same compositions were all virtually the same as that of the 1:1:10 glass.
An 8 mole % TiO2 composition was initially chosen to prove that crystalline deposits could be air plasma sprayed (APS). APS is important with respect to coating the low expansion, silica based materials such as CERADYNE 220, which is preferred for the SPF dies. Surface qualities improved with preheat temperature and the coatings were crystalline when preheat temperatures exceeded 700xc2x0 C. A crystalline coating of low thermal expansion indialite could also be obtained by post surface heat treatment using the plasma torch. Turning the powder feed off and passing the plasma flame over the coated surface provided sufficient heat to crystallize the coating. Coating materials were tested by decreasing the titania content to achieve a better match of the CTE with that of CERADYNE 220.